The mechanism of orthostatic and haemorrhagic fainting

Abstract

In order to study the adjustment of central circulation to postural changes and the mechanism of orthostatic fainting, the pressure in the brachial artery, the pulmonary artery, the right ventricle and heart rate (HR) were recorded in sixteen healthy young men, both supine and after tilting to 45 degrees and 90 degrees head up, before (normovolaemic, NV) as well as after (hypovolaemic, HV) withdrawal and reinfusion of (mostly) 950 g blood (about 15% of blood volume, BV). Two subjects fainted in supine HV, two in 90 degrees NV, four disclosed impending symptoms of fainting, and two fainted in 90 degrees HV. 'Fainters' differed from the others by smaller BV in relation to body height, higher HR in 90 degrees NV and lower arterial mean pressure in HV. In the three fainting situations, right ventricular enddiastolic pressure (PRveD) was markedly lowered to or below 0 mmHg. Arterial diastolic pressure (PaD) was not correlated with PRveD, but the pulse pressure decreased with the fall in PRveD. In 90 degrees HV, PaD fell in the six subjects who fainted or disclosed impending symptoms. Irrespective of the situation, the fainting attack involved a sudden decrease of HR and arterial pressure, concomitantly with a first unchanged then increased PRveD and/or pulmonary artery diastolic pressure. A reflex control of the filling volume/pressure of the heart is assumed to precipitate fainting by counteracting the arterial blood pressure regulation.

Norepinephrine and Epinephrine Responses during Orthostatic Intolerance in Healthy Elderly Men

“…Unlike results in younger individuals [31], in the present study, blood volume (as estimated from height 3 and body mass) was also not significantly different between the HYPO and NORMO groups. Furthermore, the percentage change in plasma volume during 90°H UT was also not significantly different between groups.…”

“…It has been suggested that susceptibility to fainting is increased with lower blood volume [31,32] and that the magnitude of central hypovolemia determines the cardiovascular responses to orthostatic stress [2,21]. Unlike results in younger individuals [31], in the present study, blood volume (as estimated from height 3 and body mass) was also not significantly different between the HYPO and NORMO groups.…”

The venous system is the main determinant of hypotension in patients with vasovagal syncope

“…The reduction in SV that we and other authors 11,17,32,35 observed before loss of consciousness might be related not only to a reduced venous return but also to an impairment of myocardial contractility. In this regard, the literature offers rather contrasting data.…”

Cardiovascular Adjustments to Gravitational Stress

“…Measurable orthostatic intolerance appeared after an acute blood volume loss of 500 ml. Bergenwald et al (19) studied normal subjects during prolonged head-up tilt and found that fainters had significantly lower (-8%) total blood volume than nonfainters. The size of the central blood volume pool may also be an important determinant.…”